Japanese Unexamined Patent Application Publication No. 2009-153234 suggests a technology for selecting a threshold voltage in an overcurrent control circuit. The threshold voltage is selected according to operating modes, normal output mode and low load mode. An output voltage of a transformer on the secondary side is high in normal output mode and low in low-load mode, for example, on standby. A voltage induced in a primary auxiliary winding connected to a power-supply terminal VCC of a power-supply control IC corresponds to a voltage calculated by multiplying the output voltage of the power supply unit by a turn ratio of the transformer. Therefore, the output voltage of the power supply and the auxiliary winding have a proportional relationship. The switching power supply in the above document changes the threshold voltage applied to an overcurrent control comparator according to variations in the supply voltage VCC. Specifically, the threshold voltage in low load mode is set lower than the threshold voltage in normal output mode. With this configuration, a peak current of a MOSFET in low load mode is regulated to a lower level than that in normal output mode.
If the output voltage of the switching power supply in low load mode is 5 V and that in normal output mode is 24 V, the output voltage in normal output mode is 4.8 times higher than that in low load mode. In general, the supply voltage VCC of the power supply control IC is 14 V or higher. Therefore, the voltage induced in the primary auxiliary winding needs to be set to 15 V, for example. In normal output mode, the voltage induced in the primary auxiliary winding is 72 V. Namely, the voltage higher than a tolerance that is about 20 V is applied to the power supply control IC.
A step-down circuit may be connected between the primary auxiliary winding and the power supply control IC to reduce the voltage below the tolerance of the power supply control IC so that the voltage higher than the tolerance is not applied. However, if the voltage is reduced, a selection range of the threshold voltage (hereinafter also referred to as a reference voltage) for overcurrent detection is narrowed. This is because the selection range is defined according to a ratio of variations in the supply voltage VCC, that is, the selection range varies proportional to variations in the supply voltage VCC. If the power supply control IC does not have a voltage limitation, the supply voltage VCC is 15 V in low load mode and 72 V in normal output mode. Namely, the supply voltage VCC in normal output mode is 4.8 times higher than that in low load mode. If the threshold voltage for overcurrent detection in low load mode is V1, the threshold voltage in normal output mode is V1×4.8. Namely, the threshold voltage can be varied from the level in low load mode to the level in normal output mode, which is 4.8 times higher than the level in low load mode. The selection range is a “4.8-time” selection range. However, the power supply control IC actually has a voltage limitation. The supply voltage VCC of the power-supply IC is limited to 20 V in normal output mode. Therefore, the supply voltage VCC in normal output mode is only about 1.3 times higher than that in low load mode (20 V/15 V≈1.3). If the threshold voltage in low load mode is V1, the threshold voltage in normal output mode is V1×1.3. Namely, the selection range is a “1.3-time” selection range.
Therefore, there is a need to provide a technology for achieving a wide selection range of reference voltages for overcurrent detection in a power supply system in which the different reference voltages are set for different modes, respectively.